The mineralogy of the processes leading to the formation of soils by the tropical weathering and leaching of rocks is reviewed, and illustrated by the results of an investigation into the formation, mineralogy and plasticity properties of some soils formed on basalt, granite, shale and sandstone in west Malaysia.
It is shown that hydrolysis followed by differential solution can account for the formation of the samples studied. Kaolinite was always present and generally the dominant clay mineral, although it gave place to gibbsite under severe weathering, and illite was also present in some of the less weathered soils on granite and shale. Quartz was prominent in soils on granite and sedimentary rocks, which are rich in this mineral. Iron oxide generally occurred, usually as goethite in the soils but as hematite in the nodules which were generally present in the soils selected for study, although gibbsite (hydrated aluminium oxide) was the main constituent of the nodules in one case. The observed mineralogical composition and clay content of the soils are consistent with the range of plasticity properties observed. The activity of some soils was reduced by the presence of gibbsite or goethite in the clay-sized fraction.